This invention relates to the production of ion beams, and, more particularly, to an ion evaporation source for tin ions.
Liquid metal ion sources provide high current densities of metallic ions from a source having a small virtual source size. Such high brightness and small source size are required when the ion beam is to be focused with a high resolution of, for example, less than one micrometer spot size, and utilized in applications such as fabrication of semiconductor microcircuits. The high current density and small virtual source size are achieved by emitting the ions from a substrate having a sharp point, such as the point of a needle. In one such approach, a needle is covered with a layer of liquid ion source metal, and a cusp in the liquid metal at the point of the needle is created by application of an electrostatic field. This fine cusp then becomes the emitting source for evaporation of the ions. As the ions are emitted from the source, more liquid metal must flow from a reservoir down the needle to the cusp, to replenish that evaporated.
For this type of high brightness ion source to operate properly, the ion source metal must wet the needle to ensure a smooth flow of metal from the reservoir to the cusp. If the ion source metal does not wet the needle, or wets the needle incompletely, the source metal alloy may form balls or lumps along the surface of the needle, thereby interfering with the metal flow, preventing the formation of the cusp, and increasing the apparent source size, with the result that the emitted ion beam cannot be properly focused.
The needle of the evaporation source is typically made from a metal having sufficient ductility that it can be formed into the shape of a needle, but of sufficient resistance to degradation in the liquid metal of the ion source that it will have a long life. Tungsten is a commonly used material for the needle, and is also used in the combined heater and reservoir that heats the needle and holds the liquid metal that flows to the needle tip as the source operates. Rhenium, molybdenum, and other refractory metals may also be utilized.
One of the increasingly important ions for which an evaporation source is needed is tin. Beams of tin ions having large numbers of ions per unit area cross section of the beam are required for applications such as the doping of indium phosphide used in heterostructures. In such applications, a finely focused beam of tin ions, preferably Sn.sup.+ or Sn.sup.++, deposits the ions in patterns of the host structure, to achieve particular electronic effects.
The preparation of a tin ion evaporation source has not heretofore been possible. Tin does not wet tungsten or other candidate evaporation element materials readily at temperatures near to the melting point of the tin. It is desirable to operate at such temperatures just above the melting point, to prolong the life of the source by avoiding burnout of the source element, and to prevent overly rapid evaporation of the tin in the ion column. Even if the operating temperature of the ion evaporation source is raised far above the desirable operating level, the source still does not run well to produce a uniform, fine beam of tin ions.
consequently, there is a need for an ion evaporation source for tin ions. The approach should permit stable, long-life operation of the source at temperatures not far above the melting point of tin. The present invention fulfills this need, and further provides related advantages.